The effects of nitrogen on the physicochemical properties of cold acetylene plasmas have been experimentally studied in C2H2/Ar capacitive radio frequency (RF) discharges. Two discharges containing respectively 0.8% and 6.3% N2 in the initial gas mixture were investigated under conditions of incipient polymerization, using mass spectrometry, light scattering, and optical emission spectroscopy. During the initial transient, dust particles, small polyynes, and C2 radicals were found to form and decay in parallel with a steep drop in the C2H2 concentration and an increase in the concentration of hydrogen cyanide, which was more prominent for the 6.3% N2 mixture, and participated actively in the plasma chemistry. Over this time interval, relevant plasma parameters including the electron density and excitation temperature, the self-bias voltage, and the density of electronic excited states of various plasma species were found to undergo sharp variations. After this initial transient, lasting for about 20 s, a steady state was reached with stable plasma properties. Ion distributions were measured in the steady state, where no dust particles remained. The distributions of positive ions were dominated by species with an even number of carbon atoms, reflecting the prevailing polymerization mechanisms. The increase in the N2 concentration from 0.8% to 6.3% led to a decrease in the global cation signal and to a marked growth in the intensity of detected anions. The cation distributions did not change much, but in the anion distributions, the peaks corresponding to the masses of the C2n−1N− (n = 1–4) ions grew by orders of magnitude, in contrast with those of the adjacent C2nH− peaks, which showed comparatively modest changes. These results could help identify anionic polymerization routes. Note that the two anion families mentioned correspond to the negative ions observed thus far in interstellar and circumstellar media.
窒素が低温アセチレンプラズマの物理化学的性質に及ぼす影響を、C2H2/Ar容量結合型高周波(RF)放電において実験的に研究した。初期混合ガス中にそれぞれ0.8%および6.3%のN2を含む2つの放電を、初期重合の条件下で、質量分析、光散乱、および発光分光法を用いて調査した。初期過渡状態の間に、ダスト粒子、小さなポリイン類、およびC2ラジカルが、C2H2濃度の急激な低下とシアン化水素濃度の増加と並行して生成・消滅することが見いだされた。このシアン化水素濃度の増加は、6.3%N2混合ガスの場合により顕著であり、プラズマ化学に積極的に関与した。この時間間隔の間に、電子密度と励起温度、自己バイアス電圧、および様々なプラズマ種の電子励起状態の密度を含む関連プラズマパラメータが急激な変動を受けることが見いだされた。約20秒間続くこの初期過渡状態の後、安定したプラズマ特性を備えた定常状態に達した。イオン分布は、ダスト粒子が残っていない定常状態で測定された。陽イオンの分布は、偶数の炭素原子数を持つ種が優勢であり、支配的な重合機構を反映していた。N2濃度の0.8%から6.3%への増加は、全陽イオンシグナルの減少と、検出された陰イオンの強度の顕著な増大をもたらした。陽イオン分布はあまり変化しなかったが、陰イオン分布では、C2n−1N− (n = 1–4) イオンの